The factors controlling the selectivities of nucleophiles and electrophiles in cation-nucleophile combination, nucleophilic aromatic substitution, and SN2 type reactions will be investigated. Primary attention will be devoted to the existence of selectivity-reactivity relationships with peripheral attention to polarizability and steric effects on selectivities. The information obtained from the carefully controlled reaction series studied will be appled to the problem of disentangling the complex mechanisms of multi-step reactions such as those commonly found in biological systems.